1. Field of the Invention
This invention relates to vapor phase epoxidation reactions of ethylene with molecular oxygen; and, more particularly to epoxidation reactions using specific cesium-promoted silver catalysts.
2. Description of the Prior Art
The preparation of ethylene oxide by the oxidation of ethylene in the presence of suitable catalysts is well known. These known processes can be generally separated into two groups; the first utilizes air and the second utilizes molecular oxygen, e.g., from about 85 mol percent to about 99 mol percent. "Silver catalysts" are utilized with both groups.
Although the first reference to the use of silver as such a catalyst was made by Walter in British Pat. No. 21,941 (1905), it was not until some thirty years later that the first disclosures were made of the use of silver as a catalyst in the vapor phase oxidation of ethylene to ethylene oxide. See Societe Francaise de Catalyse Generalisee, French Pat. No. 729,952 (1932); and Lefort, U.S. Pat. No. 1,998,878 (1935).
Since silver is expensive, optimizing the amount of silver employed in a supported catalyst for a desired conversion and selectivity to products has been widely investigated. A variety of techniques have been developed for the depositing of relatively small, but highly active amounts of silver on surfaces of non-silver supports such as alumina. For example, McKim and Cambron in Canadian Journal of Research, Volume 27, Section B (1949), pp. 813-827, described a method for depositing particulate silver on a support by decomposing silver oxalate in aqueous ethanolamine at 60.degree. C. and forming a paste which is applied to the surface of the support. In U.S. Pat. No. 3,043,854 issued July 10, 1962 to Endler, a silver coating formed by decomposition of a silver carbonate slurry is applied to a catalyst support surface.
Recently it has been disclosed that supported silver catalysts can be prepared by impregnating a porous substrate with certain silver containing solutions and evaporating or decomposing the solutions to deposit silver on the substrate. U.S. Pat. No. 3,702,259 to Nielsen describes the use of an aqueous silver oxalate impregnating solution which employes a solubilizing/reducing agent of ethylenediamine, a mixture of ethylenediamine or ethanolamine and ammonia, or a mixture of ethylenediamine and ethanolamine. Van Bylandtlaan, in Belgium Pat. No. 808,278 (1974) employs an aqueous solution of hexamethylenetetramine with an ethylenediamine silver complex to deposit silver on an alumina support by decomposition. Additionally, it has been disclosed in Japanese Pat. No. 71/19,606 to Fujii et al that impregnation of inorganic supports with aqueous silver nitrate/alkanolamine complexes with subsequent thermal decomposition gives supported silver catalysts for ethylene epoxidation.
Recently it has been disclosed in British Pat. No. 1,413,251 to Nielsen and La Rochelle that certain alkali metals, such as cesium, can be deposited on a refractory support coincidentally with the silver metal (U.S. Pat. No. 3,962,136).
Surprisingly, it has now been discovered that certain cesium-promoted silver catalysts are extremely stable, physically durable and more selective in specific ethylene oxidation processes than heretofore known promoted catalysts.
The promoted silver catalyst is easily prepared by impregnating a porous, inorganic substrate with an impregnating solution comprising a solvent, a silver carboxylate, certain amines and a salt of cesium. The impregnated support is heated to decompose the complex thus depositing the silver on the substrate and activating the catalyst.
It has been found that the instant promoted catalysts provide outstanding selectivity in air oxidation processes or oxygen processes using nitrogen or other inert gas ballast. This is particularly important in that such air processes are not closed systems and some proportion of the unreacted ethylene is lost by venting excess gas. Additionally, the instant catalysts show high attrition resistance and surprisingly high mechanical strength.